Debugger confuses variables

Consider the following example:

import Control.Exception

f = do
  ma <- try $ evaluate a
  case ma of
    Right str -> return a
    Left  err -> return $ show (err :: SomeException)
  where
    a :: String
    a = error "hi"

Load into ghci, set a breakpoint on the 5th line, then run f:

:break 5
f

You will get

Stopped at Ex.hs:(5,3)-(7,53)
_result :: IO String = _
a :: Either SomeException String = Left _

This doesn't make sense. a has type String; it is ma that has type Either SomeException String.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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